Vehicles with diesel engines typically get 25 to 30 percent more miles to the gallon than their gasoline counterparts, according to Charlie Freese, executive engineering director at GM Powertrain. Freese said the many factors that make diesel engines more efficient include operating unthrottled and more efficient oxidizing of fuel. Diesel engines also have a higher compression ratio, and the heavier diesel fuel has a higher energy density, according to Freese